Firefighters in the western U.S. state of California say they are starting to make progress in fighting a massive wildfire that is threatening a famous national park.

The firefighters say a night of cooling temperatures and higher humidity has helped them to contain about the third of the blaze, which is located on the edge of Yosemite National Park.

Nearly 5,000 firefighters have been battling the Rim Fire for two weeks. While the massive blaze has burned more than 800-square kilometers, including inside Yosemite, the fire is still some distance from the park's major attractions, including granite rock formations and waterfalls.

The wildfire is the largest one in the United States this year, and the fifth largest California wildfire on record.

The U.S. Forest Service says firefighters expect to fully contain the Rim Fire by September 20.

The fire has destroyed more than 100 homes, businesses and other structures. Authorities say about 4,500 other structures are threatened. Mandatory evacuations are in place in at least two areas.

Authorities have been using an unmanned military Predator drone, similar to those used in Afghanistan, to provide information on new fires they would otherwise not be able to see.

The cause of the fire that broke out August 17 is under investigation. It is one of dozens of large wildfires burning in the western United States, which has been plagued by a lack of snow and rainfall this year.